1. Field of the Invention
The present invention relates generally to heat transfer surfaces and the method by which such a surface may be manufactured. In particular, the invention relates to a porous surface for efficiently boiling a liquid such as a liquid refrigerant and to the method for flame spraying and processing a metal substrate to produce such a surface.
2. Description of the Prior Art
It is well known that one of the most effective mechanisms for transferring heat from a heated surface to a liquid in contact with the surface is nucleate boiling. In the nucleate boiling process, heat transferred from the heated surface vaporizes liquid in contact and bubbles are formed. Vapor trapped in a bubble is superheated by heat from the surface and the bubble grows in size. When the bubble size is sufficient, surface tension is overcome and the bubble breaks free of the surface. As the bubble leaves the surface, liquid enters the volume vacated by the bubble and vapor remaining in the volume has a source of additional liquid to vaporize to form another bubble. The continual forming of bubbles at the surface, the release of the bubbles from the surface and the rewetting of the surface together with the convective effect of the vapor bubbles rising through and mixing the liquid result in an improved heat transfer rate for the heat transfer surface.
It is also well known that the nucleate boiling process can be enhanced by configuring the heat transfer surface so that it has nucleation sites that provide locations for the entrapment of vapor and promote the formation of vapor bubbles. Simply roughening a heat transfer surface, for example, will provide nucleation sites that can improve the heat transfer characteristics of the surface over a similar smooth surface.
In boiling liquid refrigerants, for example in the evaporator of an air conditioning or refrigeration system, nucleation sites of the re-entrant type produce stable bubble columns and good surface heat transfer characteristics. A re-entrant type nucleation site is a surface cavity in which the opening of the cavity is smaller than the subsurface volume of the cavity. An excessive influx of the surrounding liquid can flood a re-entrant type nucleation site and deactivate it. By configuring the heat transfer surface so that it has relatively larger communicating subsurface channels with relatively smaller openings to the surface, flooding of the vapor entrapment or nucleation sites can be prevented and the heat transfer characteristics of the surface improved.
Over the years, in recognition of the above principles, many efforts have been made to produce heat transfer surfaces of improved efficiency having subsurface nucleation sites.
One method of manufacturing such a surface is by machining, rolling or milling. Several of such methods are disclosed in U.S. Pat. No. 3,696,861, U.S. Pat. No. 3,768,290, U.S. Pat. No. 4,159,739 and U.S. Pat. No. 4,438,807. These methods, however, do not lend themselves to the manufacture of a heat transfer surface on a substrate of a hard metal such as titanium.
Another method is disclosed in U.S. Pat. No. 4,129,181, in which a metal surface is prepared by first applying a reticulated organic foam layer then plating a thin metal coating on the foam substrate. The foam layer is then pyrolyzed at a temperature in the range of 575.degree.-980.degree. F. This heating can anneal the metal, resulting in degradation of its mechanical properties.
Flame spraying metallic particles on a metal substrate is another method of manufacture. Several variations of that technique have been developed and disclosed. In the method disclosed in U.S. Pat. No. 3,990,862, the oxidizer-fuel gas balance is of prime importance. In the method disclosed in U.S. Pat. No. 4,354,550, the surface must be preheated before being flame sprayed. In the method disclosed in U.S. Pat. No. 4,753,849, issued to the inventor of the present invention, two dissimilar metals are flame sprayed on to a metal substrate. One of the metals is then etched out by an acid bath to form subsurface cavities in the substrate surface.
The method disclosed in U.S. Pat. No. 4,359,086 combines machining and flame spraying techniques by first rolling and milling a surface then flame spraying the machined surfaces to form a porous coating over the machined channels on the surface.
There is, therefore, a need for a high efficiency heat transfer surface for boiling liquids that can be manufactured simply, economically and safely.